. This proposal will use a yeast model system to explore a checkpoint that coordinates completion of DNA replication with mitosis. Specifically, this proposal will investigate how a yeast cell, sensing that replication is not yet complete, blocks the transition from metaphase to anaphase. The importance of this checkpoint is obvious, in that entry into the process of cell division prior to duplication of the genetic material is a catastrophic event, leading to instantaneous loss of genomic integrity. The proposal focuses on several conserved proteins that were identified through genetic analysis, some of which previously had not been associated with checkpoint functions. The specific aims are to characterize the parameters of a "late" S phase checkpoint that depends on Pds1 to delay anaphase, to investigate the role of Rad23 and Ddi1 in the Pds1-dependent S phase checkpoint and to engage in a detailed structure/function analysis of the Rad23 and Ddi1 proteins.